1. Field of the Invention
This invention relates to an apparatus and method for dry forming fibrous pads, and specifically, to apparatus and method for forming fibrous pads suitable for use by themselves, or in conjunction with other components to form sanitary products such as disposable diapers, sanitary napkins and the like.
2. Description of the Prior Art
Apparatus and method for forming absorbent fibrous pads are known in the prior art. Such fibrous pads are often formed from 100% woodpulp fibers, and are utilized as internal absorbent components in sanitary products such as disposable diapers, sanitary napkins and the like. In many of these applications the fibrous pads are required to have a relatively high basis weight. For example, the center region of the absorbent pad shown in FIG. 3 of U.S. Pat. No. 3,766,922 issued on Oct. 23, 1973, and assigned to Scott Paper Company, has a basis weight of approximately 45 oz/yd..sup.2.
It has been suggested to dry form fibrous pads directly into three-dimensional pockets, or cavities of pad forming members which close the lower end of a pad forming area in conventional dry forming equipment. The pad forming member can be in the form of a cylindrical condenser roll having a three-dimensional pocket formed therein, or alternatively, can be in the form of a foraminous belt having three-dimensional pockets formed therein. In these prior art apparatus the fibrous pads are directly formed into the pockets by applying a vacuum through only the lower, or bottom surface of the pockets, i.e., only the bottom surface is foraminous. When the pad forming member is in the form of a cylindrical condenser roll the side walls of each three-dimensional pocket converge toward each other in a direction from the outer periphery of the condenser roll to the bottom surface, and accordingly, the surface area of the bottom surface of each pocket is less than the surface area of the opening into it.
In other prior art apparatus and methods fibrous pads are directly formed on a cavity-free outer foraminous surface of a cylindrical condenser roll or foraminous belt.
Accordingly, in the prior art apparatus and methods the foraminous surface through which the vacuum is applied either has substantially the same area (the outer surface of a condenser roll or belt), or a lesser area (the bottom surface of a pocket in a condenser roll) than the area of the fibrous pad which ultimately is to be formed.
When fibers are initially deposited as a layer on a foraminous surface from an air suspension they establish increased resistance to air flow through said surface. This resistance to air flow increases significantly with slight increases in basis weight of the fibrous layer formed on the foraminous surface. This resistance to air flow can become so great as to prevent the required volume of air containing suspended fibers to be pulled through the foraminous surface to form a fibrous pad having a desired basis weight. Even the use of uneconomical, high horsepower vacuum pumps to establish high vacuum levels may not solve the problem, since the high vacuum levels may merely consolidate the fibers into an air-impervious layer on the foraminous surface before the requisite weight of fibers is deposited in said cavity. When the vacuum level is adjusted to avoid excessive compaction of the fibrous layer the large resistance to air flow through the layer still makes it necessary to employ a long formation area to form fibrous pads of a high basis weight. Accordingly, excessively large equipment taking up valuable plant space may be required. The instant invention overcomes the problems set forth in this paragraph.